Conventionally, there is studied and developed, in various organizations, a surface light emitting device employing an organic electroluminescent element (hereinafter referred to as “organic EL element”).
For example, an organic EL element has a laminated structure including a transparent electrode serving as an anode, a hole transport layer, a light emitting layer (an organic light emitting layer), an electron injection layer, and an electrode serving as a cathode, which are stacked in this order and provided on one side of a light transmitting substrate (transparent substrate). With regard to the organic EL element with such a laminated structure, a voltage applied between the anode and the cathode causes generation of light in the light emitting layer. Light generated at the light emitting layer is emitted outside via the transparent electrode and the light transmitting substrate.
The organic EL element is designed to give a self-emission light in various wavelengths, with a relatively high yield. Such organic EL elements are expected to be applied for production of displaying apparatuses (e.g., light emitters used for such as flat panel displays), and light sources (e.g., liquid-crystal displaying backlights and illuminating light sources). Some of organic EL elements have already been developed for practical uses.
Recently, in consideration of application and development of organic EL elements to such uses, an organic EL element having high efficiency, prolonged lifetime, and high brightness is expected.
It is considered that the efficiency of the organic EL element is mainly dominated by three of electrical-optical conversion efficiency, driving voltage, and light extraction efficiency.
With regard to the electrical-optical conversion efficiency, it was reported that the organic EL element with the light emitting layer made of phosphorescent light emitting material can have external quantum efficiency greater than 20%. The external quantum efficiency of 20% is considered to be corresponding to internal quantum efficiency of about 100%. It is considered that the organic EL element having the electrical-optical conversion efficiency reaching a limiting value has been developed. In view of the driving voltage, an organic EL element which shows relatively high brightness in receipt of voltage higher by 10 to 20% than voltage corresponding to an energy gap of the light emitting layer has been developed. Consequently, it is expected that improvement of these two factors (electrical-optical conversion) are not so effective for an increase in the efficiency of the organic EL element.
Generally, the light extraction efficiency of the organic EL element is about 20 to 30% (this value is slightly varied depending on lighting patterns, and/or a layer structure between the anode and the cathode). since material constituting a light emitting part and a surrounding part thereof has characteristics (such as a high refractive index and light absorption properties), total reflection at an interface between materials having different refractive indices and light absorption caused by materials are likely to inhibit effective transmission of light to an outside as a light emission observation side. As a result, it is considered that the light extraction efficiency shows such low a value. In brief, the light extraction efficiency of 20 to 30% means 70 to 80% of total amount of emitted light is dominated by light which does not effectively contribute to light emission. Consequently, it is considered that improvement of the light extraction efficiency causes a great increase in the efficiency of the organic EL element.
In consideration of the above background, with regard to the field of the organic EL element, there is studied and developed, in various organizations, to improve the light extraction efficiency of the organic EL element. Especially, there have been many efforts to increase light which is emitted from the light emitting layer and reaches the light transmitting substrate. With regard to an organic EL element, the light emitting layer has a refractive index of about 1.7, and ITO which is common material of the transparent electrode has a refractive index of about 1.8 to 2.0, and a glass substrate (e.g., a soda lime glass substrate and a non-alkali glass substrate) which is common material of the light transmitting substrate has a refractive index of about 1.5. Consequently, even when the transparent electrode has a refractive index of 1.7, a loss caused by total reflection at the interface between the transparent electrode and the light transmitting substrate reaches about 50% of totally reflected light. The value of about 50% is calculated by use of point source approximation in consideration that the emitted light is expressed as an integration of three dimensional radiation of light from organic molecules.
Consequently, in the organic EL element, with decreasing a loss caused by total reflection between the light emitting layer and the light transmitting substrate, it is possible to greatly improve the light extraction efficiency.
The most simple and effective approach for reducing the total reflection loss between the light emitting layer and the light transmitting substrate is to decrease a refractive index difference at an interface existing between the light emitting layer and the light transmitting substrate. In this approach, two efforts to decrease the refractive index of the light emitting layer and increase the refractive index of the light transmitting substrate are considered. With regard to the former effort, available material is limited, and some material may cause a great decrease in the light emission efficiency and lifetime. It is therefore now difficult to improve the light extraction efficiency in line with the former effort. Meanwhile, with regard to the latter effort, it is known that use of a high refractive index material glass substrate of a refractive index of 1.85 as the light transmitting substrate constituting the organic EL element may improve the light extraction efficiency (e.g., see document 1 (U.S. Pat. No. 7,053,547 B2)). Further, it is known that a plastic substrate which is provided with a gas barrier layer with gas barrier properties of blocking gas (e.g., oxygen and moisture) and has a refractive index higher than that of a general glass substrate is used as the light transmitting substrate (see document 2 (U.S. Pat. No. 5,693,956 B2) and document 3 (JP 2004-322489 A)). According to techniques disclosed in documents 2 and 3, it is possible to improve the light extraction efficiency in addition to waterproof properties. The light emitting device disclosed in document 2 has a laminated structure mounted on a barrier layer formed on a first surface of a plastic substrate. The laminated structure includes an anode, a light emitting layer, and a cathode. The laminated structure is covered with a protection part made of epoxy resin and a medium constituting a dielectric layer is interposed between the laminated structure and the protection part. The light emitting device is designed to emit light via a second surface of the plastic substrate.
Further, there has been proposed an organic EL element having an improved effect for suppressing element deterioration caused by gas (e.g., water vapor). In this organic EL element, a laminated structure including a transparent anode layer, a light emitting medium layer, and a cathode which are stacked on a plastic substrate in this order is hermetically sealed in a housing constituted by a glass substrate and a moisture resistance film (see document 4 (JP 2002-373777 A)). In the organic EL element disclosed in document 4, the plastic substrate is designed to have water content not greater than 0.2% by weight. Further, document 4 discloses that forming a gas barrier layer on a first surface (surface in contact with the transparent anode) of the plastic substrate or the first surface and a second surface of the plastic substrate can more improve the effect of suppressing element deterioration.
With regard to the organic EL element employing the high refractive index glass substrate as disclosed in document 1, since the high refractive index glass material is expensive, industrial availability thereof is low in the present circumstances. Additionally, the high refractive index glass substrate generally contains various impurities (e.g., heavy metal). Thus, many of the high refractive index glass substrates are fragile and have insufficient weatherproof properties.
According to the organic EL element employing the light transmitting substrate constituted by the plastic substrate provided with the barrier layer as disclosed in documents 2 and 3, it is possible to reduce the production cost relative to the instance employing the high refractive index glass material. However, with regard to the organic EL element disclosed in documents 2 and 3, the second surface of the plastic substrate used as a light extraction surface easily suffers from scratches. Further, organic material has a lowered weatherproof property and a lowered ultraviolet resistance relative to glass. Thus, when the organic EL element is used outside, deterioration of long-time reliability of plastic substrate and the light emitting layer is likely to occur. Moreover, the plastic substrate provided with the barrier layer is expensive relative to a general plastic substrate devoid of a barrier layer, and therefore use of the plastic substrate provided with the barrier layer has a disadvantage in cost.
With regard to the organic EL element disclosed in the aforementioned patent document 4, the number of the interfaces (refractive index interfaces) existing between the light emitting medium layer and the air (air in the light extraction side) is increased. Thus, the total reflection loss and the Fresnel loss are increased, and therefore the light extraction efficiency is decreased.